In patients with HIV and other immunosuppressive diseases, one strategy for preventing infectious processes has been chemoprophylaxis or long term suppression. However, drug toxicity and development of pathogen resistance has made this less attractive, as has the complexity of resulting drug regimens. Another approach would be detected of signal in serum or buffy coats by PCR for pathogens such as MAX, CMV, TB, PCP, and perhaps others. PCR is being adapted for rapid screening. Initial efforts will involve CMV and MAC. Laboratory samples are being tested and quantitative standards developed. A second stage of development will involve human samples. A third stage will involve prospective monitoring of patients. This strategy is a major change from current emphasis on expensive and toxic drugs which are difficult to comply with. Proper probes, sample types, and populations need to be defined.